1. Field of the Invention
The present invention relates to composite piezoelectric ceramics and piezoelectric devices.
2. Description of the Related Art
Piezoelectric ceramics produce a mechanical strain and a stress in response to the application of an electric field. Piezoelectric ceramics showing such a piezoelectric phenomenon are used in various piezoelectric devices such as actuators, piezoelectric oscillations, speakers and sensors. Further, the materials have recently found use in vibration power generation.
Piezoelectric ceramic actuators have characteristics such as the ability to produce slight displacements with high precision and the ability to generate a high stress. For example, the piezoelectric ceramic actuators are used for positioning in precision machine tools and optical apparatuses. The most frequent piezoelectric ceramic used in such actuators is lead zirconate titanate (PZT) having excellent piezoelectricity. However, the fact that PZT contains a large amount of lead (Pb) has recently given rise to a concern about its adverse effects on global environment such as the elution of Pb by acid rain. Thus, there has been a demand that PZT be replaced by a piezoelectric ceramic material having a sufficiently decreased amount of Pb. In response to such needs, various Pb-free piezoelectric ceramic materials have been proposed.
Barium titanate (BaTiO3) and potassium niobate (KNbO3) are known Pb-free piezoelectric ceramic materials. To obtain improved piezoelectric characteristics, piezoelectric ceramic materials that are solid solutions including BaTiO3 and KNbO3 have been proposed.
For example, a piezoelectric ceramic composition represented by the chemical formula x(Ba1-aCaa)TiO3-yKNbO3-zNaNbO3 has been proposed (see Japanese Unexamined Patent Application Publication No. 2003-252681).
A piezoelectric ceramic that is a solid solution of plural components is generally known to exhibit high piezoelectric characteristics due to the presence of a morphotropic phase boundary (MPB), for example, a tetragonal/orthorhombic phase boundary. Thus, crystal phase boundaries in binary or ternary solid solutions have been actively studied.
Japanese Unexamined Patent Application Publication No. 2009-227482 discloses a piezoelectric ceramic that is based on a binary solid solution containing KNbO3 and BaTiO3, the molar ratio of KNbO3 relative to the total of the two components being 0.5 to 0.9.
In a binary system such as that in Japanese Unexamined Patent Application Publication No. 2009-227482, it has been recognized that a crystal phase boundary exists in a composition containing the two components in extremely different proportions. Thus, it has been considered that piezoelectricity is exhibited only when the material has an unbalanced composition containing the components in extremely different proportions.
Further, the KNbO3/BaTiO3 binary system has a very narrow range of temperatures at which it can be sintered. Consequently, the material cannot be sintered at an excessively low sintering temperature to fail to achieve sufficient piezoelectric characteristics, whereas excessively high sintering temperatures cause problems such as variations in the composition due to the evaporation of elements such as potassium (K).
Japanese Unexamined Patent Application Publication No. 2012-254913 discloses a composite piezoelectric ceramic which has a layer including at least one of KNbO3 and NaNbO3 on the surface of a compact of BaTiO3 crystal particles.
Because the structure described in Japanese Unexamined Patent Application Publication No. 2012-254913 is based on a compact of BaTiO3 crystal particles, a tetragonal to cubic structural change occurs at near the Curie temperature (Tc) of BaTiO3, resulting in significant changes in dielectric constant and piezoelectric characteristics. Once a cubic structure has been formed at or above the Tc, it is impossible to obtain sufficient piezoelectric characteristics even after the temperature is decreased.
The Tc of BaTiO3 is as low as about 120° C. This fact disadvantageously limits the service temperature of piezoelectric ceramics composed solely of or based on BaTiO3 to 100° C. or below.
Solid solutions of BaTiO3 and other components proposed in the art compare unfavorably to Pb-containing materials in terms of piezoelectric characteristics and are incapable of producing a sufficiently large displacement. Thus, there have been demands for piezoelectric ceramics that are composed of Pb-free materials, are free from the limitation of use temperatures (in contrast to the use temperatures being limited to 100° C. or below as described above) and exhibit sufficient and excellent piezoelectric characteristics, as well as demands for processes for the production of such piezoelectric ceramics.